V(D)J recombination is a site-specific somatic recombination reaction whose end products are genes encoding cell surface receptors and antibody molecules that are central to immune system function. Proper recombination is absolutely required for development of a healthy immune system: when the reaction does not occur or is poorly controlled, there is the potential for development of immunodeficiencies, leukemias, and lymphomas. Our long-term goal is to understand the mechanisms of V(D)J recombination and its regulation, and our approach has been through a biochemical analysis of the RAG proteins. Though much progress has been made in the field of recombination in the last two decades, there are still numerous questions outstanding, among them are the issue of accessibility and the regulation of recombination in vivo. In this grant application, we describe a series of experiments that will address these issues while extending our findings that showed interaction of Rag2 with core histones and the MCM complex. Through specific aims proposed, we will: 1) Investigate the requirement and participation of MCM2-7 complex and MCM associated proteins in V(D)J recombination; 2) Study the role of Rag2 C-terminus, acidic region and PHD domain, in the different steps of V(D)J recombination as well as their participation in the various protein-protein interactions mediated by this region; 3) Analyze the RSS cleavage mechanisms that depend on Rag2 full-length using a chromatin substrate. The focus of these aims is to understand the molecular mechanism that determine regulation of V(D)J recombination by Rag2 and specifically the contribution of Rag2 C-terminus to this process.